4.1 Introduction. 4.1 Introduction

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1 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 4.1 Introduction CARBOXYLATED NITRILE RUBBER LATEX (XNBR) BASED NANOCOMPOSITES 4.1 Introduction 4.2 Experimental 4.3 Results and Discussion 4.3A Nanokaolin in XNBR 4.3B Vinyl silane grafted nanokaolin in XNBR 4.3C MWCNT in XNBR 4.3D Graphene nanoplatelets in XNBR Polymer nanocomposites (PNCs) have opened a new dimension in the field of material science because of the wide spectrum of applications of the nanofillers in polymeric systems. Elastomer nanocomposites reinforced with low volume of nanofillers significantly improve the mechanical, thermal, barrier and flame retardant properties. Carboxylated nitrile rubber (XNBR) is a terpolymer of acrylonitrile, butadiene and a carboxyl group containing monomer such as acrylic or methacrylic acid. Incorporation of carboxyl groups increases inter and intra molecular interactions, resulting in improved properties of the polymer. Owing to the high polarity of carboxyl groups present, they are regarded as polar rubbers. Reinforcing effect of silica [1], ZnO [2], organic filler from olive husk powder (OHP) [3], short jute fibre [4], hydrotalcites [5] and hygro thermally decomposed polyurethane on XNBR [6] are reported. Only few reports regarding the utilization of layered silicate in XNBR are available 95

2 Chapter 4 [7-9]. Moreover, studies on multiwalled carbon nanotubes and graphene filled XNBR composites are so far not reported in literature and hence the study is the first one of its kind. This chapter deals with the study of the effect of nano kaolin, vinylsilane grafted nanokaolin, multiwalled carbon nanotubes (MWCNT) and graphene nano platelets in the mechanical and thermal properties of the nanocomposites and the different methods used for characterization of the nanocomposites. 4.2 Experimental Materials Carboxylated nitrile butadiene rubber latex (XNBR) is CLX 530 purchased from Eliokem India Pvt. Ltd. Bombay. Specification is given in Table 2.1. Nanokaolin (Nanocaliber100) and Vinylsilane grafted nanokaolin (Nanocaliber 100V) were supplied by English Indian Clays Ltd., Veli, Thiruvananthapuram. Specification is given in Table Multiwalled carbon nanotube (MWCNT) - Baytube R 150P was obtained from Baeyer Materials Science AG (Leverkusen Germany). Specification is given in Table.2.4. Graphene nanoplatelets were purchased from Quantum materials Ltd., Bangalore. Nanokaolin is designated as C and XNBR containing 1,5,10 phr nanokaolin is referred as XNBR-1C, XNBR-5C and XNBR-10C. Vinylsilane grafted nanokaolin is represented as V and XNBR containing 1, 5, 10, phr vinylsilane grafted nanokaolin as XNBR-1V, XNBR-5V and XNBR-10V. Also, Graphene nanoplateletes are referred as graphene. 96

3 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Preparation of XNBR based nanocomposites Formulation used for the preparation of dispersions for latex compounding is given in Table 3.2. Preparation of filler dispersions are dealt in Sections and Formulation used for the preparation of XNBR compounds is given in Table 3.3 and the methods for the preparation of XNBR/clay nanocomposites and XBR/MWCNT/graphene nanocomposites are given in Section and respectively Methods Tensile strength, elongation at break, modulus and tear strength of the nanocomposites were determined as per the respective ASTM standards using a Universal Testing Machine (UTM, Shimadzu, model AG1). Swelling studies were done in methyl ethyl ketone as solvent and strain sweep analysis was performed using Rubber Process Analyzer (RPA 2000). Thermal analysis of the composites were carried on TGA (TGA Q-50)) and DSC (DSC Q100) TA instruments. Xray diffraction studies were done using Bruker AXS D8 Advance Diffractometer with CuKα radiation of wavelength 1.54 A. FTIR analysis was conducted using Thermo Nicolet, Avatar 370 model IR spectrometer. The morphology of tear fractured surfaces of XNBR based nanocomposites were studied using JEOL model JSM 6390 LV. AFM images were obtained by using a Park Model XE 100 in non contacting mode. A detailed description of the methods employed is given in Chapter Results and Discussion 4.3.A Nanokaolin in XNBR Latex Carboxylated nitrile butadiene rubber (XNBR) based nanocomposites with varying amounts of nanokaolin were prepared by latex stage mixing. 97

4 Chapter 4 Sonication of nanokaolin and the technique adopted for the preparation of the nanocomposite helped to get a uniform dispersion of clay in XNBR latex. Proper dispersion of clay particles, partial exfoliation/ intercalation of clay and interaction of clay with the polar rubber latex made nanokaolin a good reinforcing filler in XNBR latex. 4.3A.1 Mechanical properties Variation of tensile strength, elongation at break, modulus at 300% elongation and tear strength of the nanocomposites were studied at different loadings of nanokaolin. Fig.4.1 shows the variation of these properties with clay loading. It was seen that the mechanical properties increased with clay loading, reached a maximum value and then decreased.thus tensile strength increased by 110%, elongation at break by 168% and modulus by 30% at 25 phr concentration of nanokaolin. Tear strength increased continuously with the increase in concentration of nano kaolin. Fig.4.1. Variation of (A) Tensile strength and Elongation at break (B) Modulus and Tear strength of XNBR-C nanocomposite There was no significant increase in the mechanical properties of the nanocomposites at low filler loading. This might be due to the lack of 98

5 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites proper dispersion of the filler. As the filler loading increased, the surface interaction with the polymer increased, resulting in better properties [10]. At still higher loadings, aggregation of filler might have reduced the interfacial area between the polymer and clay and thus weakened the available reinforcing links [11]. Kaolin particles are in the form of hexagonal plates closely packed resembling the pages of a closed book. The external surface of kaolin comprises of hydroxyl groups on one side, oxygen on the other side and the edges of all the layers called the broken bonds surfaces [12]. There is an abundance of OH groups on the plate edges and these are considered as the major reactive sites of clay particles [13]. The clay platelets are held together by weak hydrogen bonds between the hydroxyl groups of the octahedral sheet and basal oxygen of the adjacent layer tetrahedral sheet. Polar interaction between CN and COOH group of XNBR with OH group of clay led to the increase in tensile strength of the nanocomposite. Generally rigid fillers cause a significant reduction in elongation at break, but the presence of nanokaolin increased the elongation at break. The clay particles in the form of platelets provide more interfacial contact for rubber molecules, and greater slippage of the clay layers led to higher elongation at break. The increase in modulus with the addition of clay was due to the intercalation/exfoliation behaviour of the clay layers in the nanocomposite. XRD analysis confirmed the extent of intercalation. Since there was no chemical interaction between the clay layers and rubber matrix, the increase in modulus was not very high. The uniformly dispersed clay layers blocked 99

6 Chapter 4 the propagation of cracks imparting more resistance to tear propogation resulting in increased tear strength. Sonication of nanokaolin helped to loosen the close packing of the clay platelets. During compounding and ball milling, intercalation of rubber chains into the clay galleries had taken place. The interaction between the polar hydroxyl group in clay and polar CN and COOH groups on XNBR helped in intercalation. A schematic representation of clay before and after sonication and the intercalation of polymer chain into the intergallery space is represented schematically in Fig 4.2 Fig Schematic representation of clay before and after sonication and the intercalation of polymer chain into the intergallery space 4.3A.2 Swelling studies Swelling of the vulcanized samples was carried out by immersing circular specimens of 20mm diameter and 1mm thickness in methyl ethyl ketone (MEK) until equilibrium was reached. Fig.4.3 shows the change in MEK uptake with time of filled XNBR for different concentration of nanokaolin. Absorption took place in two distiguished zones. The first zone represented initial solvent uptake. Here the swelling rate was very high due to a large concentration gradient and the polymer sample was under severe 100

7 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites solvent stress. In the second zone the swelling rate was reduced due to a decrease in the concentration gradient and finally an equilibrium state with zero concentration gradient was reached [14]. Swelling rate decreased with the increase in filler loading. This was because the platelet like morphology of nanokaolin embedded in the matrix enhanced the tortuosity of the path. But it was found that at 30 phr the solvent uptake was more than that at 25 phr loading. This might be due to the aggregation of filler at the high concentration. Due to aggregation the clay platelets might not be completely available in the rubber matrix to enhance the tortuosity of the path. Fig.4.4 shows the variation of swelling index with concentration of nanokaolin.swelling index is inversely proportional to the extent of cross linking. With the addition of clay swelling index decreased due to clay rubber interaction. Fig Sorption curves of XNBR- C nanocomposites 101

8 Chapter 4 Fig Variation of swelling index with clay loading 4.3A.3 Strain sweep analysis Strain sweep studies were conducted to study the rubber filler interactions. The variations of complex modulus with strain for XNBR latex clay nanocomposite are shown in Fig.4.5. The complex modulus values at low strains were a measure of filler polymer interaction, the high values of modulus were due to higher filler-filler or filler polymer interactions. The decrease in modulus with increase in strain was due to the disentanglement of uncured rubber molecules and the breakdown of filler rubber network produced by filler-filler interaction and filler-rubber interaction. The reason why XNBR clay nanocomposite possessed high modulii was that although both the clay-rubber interaction and the compatibility between the clay and XNBR are weak in it, the filler-filler interaction was very strong due to filler aggregates. The complex modulus value increased with clay content indicating better rubber filler interaction. The increase in modulus was due to the inclusion of rigid filler particles in the soft rubber matrix 102

9 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Fig Variation of Complex modulus with strain for XNBR-C nanocomposite 4.3A.4 Thermogravimetric analysis (TGA) Fig.4.6 shows the TGA curves of XNBR and XNBR-25C. Table 4.1 gives the thermal analysis data of XNBR and XNBR-25C. Major degradation of XNBR began at C and completed around C. Hence XNBR was stable upto C in nitrogen atmosphere. The peak of DTG curve gives the temperature corresponding to the maximum degradation (T max ). XNBR degraded in two steps with Tmax at C and C. The two major peaks corresponded to the multiple degradation steps of the butadiene part present in XNBR. This involved the elimination of acrylonitrile and butadiene part followed by the main chain scission [15]. The high residue formed might be due to the formation of stable structures through probable cyclisation reaction that acrylonitrile could undergo at elevated temperature [16]. The degradation of the nanocomposites also took place in two steps. Incorporation of clay had marked increase in onset temperature, temperatures for maximum degradation (T 1 and T 2 ) and temperature for 10%, 25% and 50% weight loss. Residue remaining for the nanocomposite was much higher than 103

10 Chapter 4 in XNBR without filler. All these indicated the enhancement in the thermal stability of the nanocomposite on addition of nanokaolin. Table 4.1. Thermal analysis results of XNBR and XNBR-25C nanocomposite Samples On set temp ( 0 C) Tmax( 0 C) T1 T2 Temp. at 10% wt.loss( 0 C) Temp. at 25% wt.loss ( 0 C) Temp. at 50%wt.loss ( 0 C) Residue (%) XNBR XBR-25C Fig TGA curves of XNBR and XNBR-25C nanocomposite 4.3A.5 Differential scanning calorimetry (DSC) DSC studies of XNBR showed T g at C. Incorporation of nano kaolin showed a marginal increase in T g as given in the Table 4.2. The effect of dispersed clay layers on the free volume of rubber influenced the Tg of rubber. This might be attributed to the interaction between the organic and inorganic phases. These interactions increased the rigidity of the soft matrix and limited the movement or motion of the polymer chain. 104

11 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Table 4.2. T g values of XNBR and XNBR-25Cnanocomposite Sample XNBR XNBR-25C T g C C 4.3A.6 X-ray diffraction analysis (XRD) Fig.4.7 shows the XRD pattern of XNBR, nanokaolin ( C ) and XNBR-25C nanocomposite. No peak was observed for pure XNBR suggesting its amorphous nature. Characteristic peaks of nanokaolin shifted from 25 0 to and to This increased d spacing from 3.56 to 3.71 A 0 and 7.05 to 8.08 A 0. The peak at 20 0 collapsed, showing that exfoliation had taken place to some extent. The dispersion of clay layers in XNBR could be considered as a bimodal structure representing both intercalated and exfoliated state [17]. The reduction in the width of the peak of the composite suggested a well ordered intercalated structure. Fig 4.7. X-ray diffraction pattern of XNBR, C and XNBR-25C 105

12 Chapter 4 Similar results were reported in literature when montmorillonite (MMT) was used as the reinforcing filler. Amit et al. [18] worked on carboxylated NBR/organomodified MMT composites and showed that intercalation is a common process from 2.5 to 10 phr organo clay mixed at C. In an NBR-organically modified MMT nanocomposite a decrease in d spacing after curing was observed [19]. S.Sadhu and A.K. Bhowmick [20] have dealt with unmodified MMT on SBR with varying styrene content and using dicumyl peroxide as the curing agent and suggested that exfoliation had taken place in all these composites. 4.3A.7 Fourier transform infrared spectroscopy (FTIR) Fig.4.8. shows the IR spectrum of XNBR, nanokaolin ( C ) and XNBR-C nanocomposite. Appearance of peaks at 2930cm -1 and 2852cm -1 in the nanocomposite corresponds to the C-H stretching vibration of CH 2 groups [ cm -1 ] in the rubber back bone. This showed the intercalation of polymer chain in the interlayer space of nanokaolin. Reduction of OH stretching vibration of nanokaolin (3696cm -1, 3625cm -1 and 3438cm -1 ) [21], disappearance of CN peak of XNBR (2238cm -1 ) and the appearace of a peak at 1640cm -1 in the composite was an indication of the formation of an amide linkage. In the formation of the nanocomposite CN group of XNBR might have undergone partial hydrolysis giving the amide. Intensity of the peak at 1440cm -1 corresponding to the OH bend in carboxylic acid of XNBR was very much reduced in the composite. This showed the interaction of OH group of clay with COOH group of XNBR. The disappearance of the sharp peak at 969cm -1 corresponding to the stretching of butadiene double bond in XNBR confirmed the decrease in unsaturation of the composite.the slight shift of the peaks in the range

13 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 1200 cm -1 corresponding to the bending of Si-OH and Al-OH group of clay showed the interaction of Si-OH and Al-OH groups of nanokaolin with COOH group of XNBR. Fig FTIR spectrum of XNBR, nano kaolin ( C ) and XNBR-C nanocomposite 4.3A.8 Scanning electron microscopy (SEM) Fig.4.9 shows the SEM photographs of the tear fractured surface of XNBR (A) without filler and (B) XNBR-25C nanocomposite. Particles seen scattered in the image of XNBR without filler might be due to ZnO. In the nanocomposite the clay platelets are thickly and uniformly dispersed in the marix. Latex blending technique for the preparation of the nanocomposite followed by ball milling of the compounded latex led to homogeneity in mixing. The interaction of filler with XNBR gave greater rubber-filler adhesion leading to decrease in the tendency of the filler particles to agglomerate. From the figure it is seen that clay platelets are 107

14 Chapter 4 uniformly dispersed in matrix. This uniformity in dispersion led to the reinforcement in properties of the nanocomposite. (A) (B) Fig SEM images of (A) XNBR and (B) XNBR -25C nanocomposite 4.3A.9 Atomic force microscopy (AFM) Fig.4.10 shows the AFM images of XNBR-25C composite. This shows the homogeneous distribution of clay particles in the matrix. The homogeneously dispersed regions have different phase contrast. The light regions are the dispersed clay particles and the dark regions are the soft rubber matrix. Some aggregated clay particles are also seen in the image. 108

15 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Fig AFM images of XNBR-C nanocomposite 4.3A.10 Conclusions Inclusion of nanokaolin in XNBR enhanced the mechanical properties of the nanocomposite.tensile strength increased by 110%, elongation at break by 168% and modulus at 300% elongation by 30%. Maximum improvement in properties was obtained at 25 phr concentration of nanokaolin showing the nanoeffect was not much pronounced in the composite. Tear strength increased continuously with the increase in concentration of nano kaolin. Thermal stability of the nanocomposite increased with the incorporation of nanokaolin, but T g remained constant. XRD analysis confirmed the intercalation of polymer chain into the inerlayer space of nanokaolin. FTIR sudy confirmed the interaction of Si-OH and Al-OH groups of nanokaolin 109

16 Chapter 4 with COOH group of XNBR. SEM and AFM analysis showed uniform dispersion of nanokaolin in XNBR. 4.3B Vinylsilane Grafted Nanokaolin in XNBR Latex Effect of vinylsilane grafted nanokaolin ( V ) on the physico mechanical properties of XNBR vulcanizates are investigated. Studies prove that silane treated nanokaolin is good at imparting reinforcement to carboxylated nitrile butadiene rubber (XNBR). On grafting, silanes react mainly with some of the OH groups on the edge surface of nanokaolin. Free hydroxyl groups on the kaolin surface forms hydrogen bonds with carboxyl (COOH) and cyano (CN) groups present in the rubber chain. The polymer chain can intercalate inside the clay galleries and the butadiene part can interact with the vinyl group through van der Waals forces. The extent of hydrogen bond formation depends on the number of hydroxyl groups available at the edges of clay surface and the number of cyano and carboxyl groups in XNBR. A schematic representation of grafting of vinyl silanes on the surface of kaolin and the interaction with the polymer chain is shown in Fig Fig Plausible mechanism showing the grafting of vinyl silanes on the surface of kaolin and its interaction with the polymer chain. A - Kaolin containing OH groups on the edge surface. B - Grafting of vinyl silane on kaolin C - Increase in inter layer space on sonication D - Intercalation of polymer chain into the interlayer space. 110

17 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 4.3B.1 Mechanical properties Fig.4.12.(A) gives the variation in tensile strength/elongation at break and (B) gives the variation in modulus at 300% elongation/tear srength, of XNBR/V nanocomposite with increase in V loading. Tensile strength, elongation at break and modulus increased by 92%, 157% and 44% respectively at 15phr loading. Incorporation of clay minerals usually improves the tensile strength of polymers [22-25]. Intercalation of rubber molecules into the inter layer space of clay resulted in the increase of d spacing. High interfacial area and good interfacial adhesion led to an increase in tensile strength. Interaction between clay and polymer occurs through the interaction of OH group of clay and polar groups of XNBR. At a concentration greater than 15phr, agglomeration of clay particles resulted in filler-filler interaction leading to a decrease in the tensile strength. Elongation at break (EB) for polymer/clay nanocomposites depend on the interfacial interactions of polymer/clay system. Polar polymers have stronger interfacial interactions with clays and their composites show better elongation at break than the pure polymer [26]. There are instances where EB decreases with the filler loading [25,27, 28]. Kader et al. [17] have reported an increase of EB in NBR/MMT composite. Sodium salt of rubber seed oil (SRSO) modified kaolin gave only 22% increase in EB in NR composite [29]. Here, vinylsilane grafted nanokaolin gave 157% increase in EB. The interaction of CN and COOH groups of the polymer with OH groups on clay and the interaction of the unsaturated vinyl groups with the double bonds of the polymer facilitated the intercalation of polymer chains in between the clay platelets. At higher clay concentration non exfoliated aggregates were 111

18 Chapter 4 present and this made the composite stiffer, with reduced elongation at break. The modulus at 300% elongation increased with filler loading upto 15 phr and then decreased. The increase was due to the higher cross link density and good distribution of filler in the matrix. The increase in tear strength with concentration showed the filler could resist the growth of crack in the nanocomposite. Fig Variation of (A) Tensile strength and Elongation at break (B) Modulus and Tear strength of XNBR-V nanocomposite 4.3B.2 Swelling studies Fig.4.13 shows the sorption curves of vulcanisates obtained by plotting Qt, mole (%) uptake per 100 g of the solvent, against time. As the clay loading increased the equilibrium solvent uptake decreased. This was due to the increased hindrance exerted by the clay particles at higher loading. The interaction of polymer and clay developed a strong interface which restricted the entry of solvent [30]. Gum had the maximum solvent uptake at equilibrium swelling. Qt value decreased drastically with the addition of clay. The swelling uptake 112

19 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites was high at the initial zone. This was because of the large concentration gradient [14]. After this the sorption rate reached a plateau corresponding to the equilibrium swelling where the concentration gradient was zero. The solvent uptake was lowest for 15 phr nanocomposite. The presence of the uniformly dispersed impermeable clay layers increased the average diffusion path and thus decreased the rate of transportation [31]. Due to aggregation of clay particles at higher concentration (20 and 25 phr) some voids were formed as seen in the SEM image (Fig.4.17) of the nanocomposite with 20phr loading. This increased the solvent absorption for the nanocomposite with 20phr and 25phr clay loading. Fig Sorption curves of XNBR and XNBR-V nanocomposites 4.3B.3 Thermo gravimetric analysis (TGA) Table 4.3 gives the thermal analysis data and Fig gives the TGA curves of XNBR, XNBR-15V and XNBR-25V. Onset temperature, peak maximum (T max ) and temperature at 50% weight loss increased with the 113

20 Chapter 4 addition of V.This indicated an increase in the thermal stability of the nanocomposite. Thermal decomposition of clay in the nanocomposite created a char layer and thus delayed the degradation [32]. At 25 phr the maximum degradation temperature (T 2 ) was lower and residue higher than that of 15phr nanocomposite. This showed that the morphology of the nanocomposite at higher concentration did not allow for maintaining a good thermal stability [33]. Table 4.3 Thermal analysis results of XNBR and XNBR-V nanocomposites Samples On set temp. ( 0 C) Tmax ( 0 C) T 1 T 2 Temp. at 50%wt.loss( 0 C) Residue % XNBR XNBR -15V XNBR- 25V Fig TGA curves of XNBR and XNBR-V nanocomposites 114

21 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 4.3B.4 Differential scanning calorimetry (DSC) Usually the interaction of polymer chains with the clay particles affects the segmental mobility of polymer matrix. The confinement of the intercalated polymer chains between the clay layers reduce the segmental mobility of the matrix and increase the glass transition temperature (T g ) of the polymer. Good rubber filler interaction shifts the glass transition temperature towards higher temperatures compared to the unfilled rubber matrix [34-36]. Table 4.4 gives the T g values of XNBR and XNBR-15V. Here T g of the nanocomposite was almost the same as that of pure XNBR. This showed that there was only a weak interaction between the less hydrophilic organomodified clay and the rubber latex. So the confinement of the intercalated polymer chain in the clay galleries did not affect the segmental mobility of the polymer matrix. Table 4.4. T g values of XNBR and XNBR-15 Vnanocomposite Sample T g XNBR XNBR -15 V B.5 Xray diffraction analysis (XRD) X-ray diffraction patterns in the 2θ range for XNBR, V and the composite with 1 phr and 15 phr V are given in Fig Vinylsilane grafted nanokaolin had diffraction peaks at 12 0 (d=7.1 A 0 ), (d=4.41a 0 ) and 25 0 (d=3.56a 0 ) while XNBR being amorphous did not show a diffraction pattern in this range. The diffraction pattern for XNBR-1V had a number of strong peaks at higher angles indicating the introduction of 115

22 Chapter 4 stacked layers of clay with a crystallographic order in the XNBR matrix [37]. At a low clay concentration the viscosity of the composite might not be enough to create adequate shearing force to intercalate the rubber molecules into the gallery space [38]. At higher loading the dispersion of clay layers in XNBR matrix might be considered as a bimodal structure representing both intercalated and exfoliated structure [17]. The peak shifted to a smaller angle side with a slight increase in d spacing to 7.8 A 0. This showed intercalation of polymer chains into the clay galleries. The position of the other two peaks remained the same. The peak at had become broader and extended to the lower 2θ value in the composite. Such broadening of diffraction peak suggested partial exfoliation. The decrease in the intensity of the peak at 25 0 showed uniform distribution of rubber particles in the inter gallery space. Literature has cited examples where introduction of hexa decyl trimethyl ammonium bromide modified montmorillonite (MMT) in NBR has increased d spacing from 2.6 nm to 3.9 nm [39]. Similar increase in d spacing by the introduction of MMT in XNBR was reported by Das, A. [18]. Partial exfoliation/intercalation in NBR/layered silicate composite was reported by Thomas,P [40]. The modified clay gave a hydrophobic environment. Sonication of clay dispersion helped in the expansion of clay layers and the polar polymer chain intercalated into the inter gallery space. Hydrogen bonding between the CN and COOH group of XNBR and OH group of clay favoured the intercalation of the polymer chain 116

23 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Fig X-ray diffraction pattern of XNBR, V and XNBR-V nanocomposites 4.3B.6 Fourier transform infrared spectroscopy (FTIR) Fig.4.16 shows the IR spectrum of XNBR and XNBR-V nanocomposite. In XNBR-V the characteristic OH stretching vibration at 3624cm -1 attributed to the inner surface hydroxyl groups of kaolin was shifted to 3612cm. -1 This gave evidence for the keying in of the polar polymer chain into the inter gallery space of vinyl grafted nano kaolin.the formation of new bands at cm -1 range also showed the presence of hydrogen bonded OH groups in the nanocomposite. XNBR-V nanocomposite showed a stronger band at 2926cm -1 and 2848 cm -1 compared to that of XNBR. This might be due to the superimposition of OH band of carboxylic acid [ ] on the CH stretching bands. The band at 2238 cm -1 remained unchanged showing there was no reaction of modified kaolin with the CN group of XNBR. The sharp peak at 1596 cm -1 gave the stretching vibration of a carboxylate ion 117

24 Chapter 4 in the composite. The carboxylate may be zinc carboxylate formed by the reaction of the COOH group in the latex with ZnO. The peak at 1445cm -1 might be due to the merging of CH 2 groups with the vinyl groups from the modified clay. Peaks in the range cm -1 showed Si-OR bonds cm -1 and 1111cm -1 peaks seen in clay were shifted to 1017 cm -1 and 1104 cm -1 respectively. The bending vibration of Al-OH in modified nanokaolin also shifted from 915 cm -1 to 909cm -1.The interaction of COOH group on XNBR with the Si-OH group and Al-OH group on the clay surface which, was already bonded to the silica tetrahedral alumina octahedral of kaolin, might have caused this shift of the Si-O and Al-OH band in the composite [18]. Corresponding to the strong peak at 970cm -1 in XNBR there was only a weak band at 965 cm -1 in the composite showing a reduction in the stretching of butadiene double bond.this might be due to the interaction of butadiene double bond with vinyl group of clay. Fig FTIR spectrum of XNBR, XNBR-Vnanocomposite and V 118

25 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 4.3B.7 Scanning electron microscopy (SEM) Fig.4.17(A),(B) and (C) shows the SEM micrographs of the tensile fractured surfaces of unfilled XNBR and composites containing 15phr and 25 phr vinyl grafted nanokaolin. In the figure the dark phase represents XNBR matrix and the bright phase corresponds to the clay particles. Clay particles are seen in the shape of platelets. At 15phr filler loading the dispersed clay particles seem to be fully encapsulated in the matrix. There was no obvious phase separation observed between clay and the matrix. This indicated better clay NBR adhesion. The uniform dispersion led to the reinforcement in the properties of the nanocomposite. At higher concentration (25phr) aggregation of clay particles created voids, due to improper dispersion of particles as seen in Fig. 4.17(C). (A) (B) (C) Fig SEM images of (A) XNBR (B) XNBR-15V (C) XNBR-25V 119

26 Chapter 4 4.3B.8 Conclusions Vinyl silane grafted nano kaolin enhanced the mechanical properties of XNBR.Tensile strength increased by 92%, elongation at break by 157% and modulus by 44%. Maximum reinforcment was obtained at 15 phr concentration of the modified nanokaolin (15V). Swelling studies also showed solvent uptake was minimum for XNBR-15V nanocomposite. Greater reduction in swelling rate was observed for XNBR-V, when compared to XNBR- C nanocomposite. This might be due to the presence of bulky vinyl groups which caused greater hindrance to the flow of solvent. Eventhough T g remained a constant, there was slight increase in the thermal stability of the nanocomposite.slight increase in interlayer spacing as evidenced by XRD analysis indicated intercalation, of polymer chains into the clay galleries. SEM analysis showed a uniform dispersion of the clay particles, at lower concentration and agglomeration of clay particles at a higher concentration. 4.3C Multiwalled Carbon Nanotube in XNBR Latex The unique mechanical properties and extremely large surface area per unit volume of carbon nanotubes makes it an intriguing reinforcement for polymers. Since the discovery of carbon nanotubes, extensive research has been done by incorporating different types of carbon nanotubes as nanoreinforcements into polymeric materials, to form new nanocomposites that possess high mechanical strength, electrical and thermal conductivity. Here, MWCNT was dispersed in sodium dodecyl benzene sulphonate (SDBS) by sonication. The dispersed MWCNT ( gm) was 120

27 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites incorporated in XNBR latex. Mechanical, electrical and thermal properties of these nano composites were studied. Nanocomposites were characterised by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Scanning electron microscope (SEM) analysis. 4.3C.1. Mechanical properties The mechanical properties (tensile strength, elongation at break, modulus at 300% elongation and tear strength) versus MWCNT concentration are graphically plotted in Fig (A) and (B). It was seen that tensile strength, elongation at break, modulus and tear strength of the nano composites increased with increase in the loading of nanotube, reached a maximum value and then decreased. Even at a very low concentration of 0.15 phr tensile strength increased by 78% and elongation at break by 86%. Modulus and tear strength increased by 31% and 19% respectively at 0.1 phr. Fig Variation of (A) Tensile strength and Elongation at break (B) Modulus and Tear strength of XNBR-MWCNT nanocomposite An efficient dispersion of nanotube and a good interphase between nanotube and the matrix led to reinforcement in properties. 121

28 Chapter 4 Sonication of nanotube dispersion in SDBS followed by latex stage mixing contributed to the uniform dispersion of nanotube in the matrix [41]. There was strong interfacial interaction between the polar nitrile (CN) group and carboxylic (COOH) group on the surface of XNBR with the electron cloud of the nanotube.the good rubber filler interaction would increase the effectiveness of stress transfer from rubber matrix to filler particles dispersed in rubber matrix. This led to an increase in the tensile strength of the nano composite. The increase in elongation at break might be due to the fact that XNBR matrix allowed more rheological flow due to good rubber filler interaction [42]. Increase in tear resistance and modulus of the nanocomposite might be due to the reinforcement of the well dispersed MWCNT with high Young s modulus and strength in XNBR matrix [43]. Formation of aggregates at higher concentration led to reduction in the mechanical properties of the nano composite. Significant improvement in mechanical properties of polymeric matrices by carbon nanotube addition has been reported. Weerachai et al. [44] observed an increase of 55% and 14% in tensile strength and elongation at break of NR-MWCNT nanocomposites at 5 phr loading. Tear resistance and modulus also increased with the increase in MWCNT loading. Bokobza, has reported an increase of 45% in elastic modulus and 70% in tensile strength by the incorporation of MWCNT in SBR. 4.3C.2 Swelling studies Fig.4.19 shows the sorption curves of the vulcanizates in MEK obtained by plotting Qt (mole percentage uptake for 100g of the solvent) 122

29 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites against Time 1/2 at room temperature. XNBR without filler showed the maximum uptake at equilibrium, since there was no restriction for the solvent to diffuse into the vulcanizate. The reduction in the equilibrium swelling is a measure of the degree of adhesion between the polymer chains and the filler particles. The presence of elecron acceptor groups in XNBR can interact strongly with the electron donating hydroxyl groups on the MWCNT surfaces [46] (IR spectrum of MWCNT, ( Fig.3.11) shows the presence of OH group in it). On interaction of the polymer with the filler, a single macromolecular chain covers sizable number of active sites on the filler surface.thus only a small number of chains are anchored on the surface of the filler [47]. It is likely that unanchored NBR chains are contributing to the swelling of the elastomeric nanocomposite network. Fig.4.20 depicts the decrease in swelling index with increase in the concentration of MWCNT. Swelling percentage is a measure of the degree of cross-linking. Reduction in swelling indicated increase of cross-link density. The effect of increase in cross link density was further confirmed by the small increase in glass transition temperatures [48] (refer Table 4.6].Weerachai Sangchay et al. have referred to the decrease in swelling in NR/MWCNT nano composite as due to the decrease in NR content by the addition of non swellable solid MWCNT[43]. 123

30 Chapter 4 Fig Sorption curves of XNBR- MWCNT nanocomposites Fig Variation of swelling index with MWCNT loading 4.3C.3 Thermogravimetric analysis (TGA) By comparing the weight loss as a function of temperature the effect of MWCNT on the thermal stability of XNBR could be analysed. Fig.4.21 shows the TGA curves of pure XNBR and XNBR MWCNT nano composites containing 0.05 and 0.15phr MWCNT. The samples displayed similar behaviour. Thermal analysis results of XNBR and the nano composites 124

31 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites are given in Table 4.5. There was an increase in the onset temperature and a shift of maximum peak (T 2 ) to higher temperature on adding MWCNT. Small increase in T 50 and T 75 also showed slight increase in the thermal stability of the nanocomposites. Percentage of residual weight increased with the increase in concentration of nanotube. The presence of residue beyond C in the TGA curve might be due to the presence of other compounding ingredients in the mix. Eventhough nanotube was added in very small quantity it could impose restriction to the mobilization of rubber macromolecules and conduct heat homogeneously and avoid heat concentration [49]. Leon D.Perez et al. [46] studied the thermal stability of MWCNT/ NBR and MWCNT/ SBR nano composites at high filler loading up to 15 phr. They suggested that the ability of MWCNT to suppress thermal degradation of the elastomer might be attributed to barrier effects as in the case of clay fillers or a reduction of the pyrolysis rate, due to the decrease of the polymer global mobility. Table 4.5. Thermal analysis results of XNBR and XNBR-MWCNT nano composite Samples Tmax( 0 C) On set temp( 0 C) T1 T2 Temp. at 50% wt.loss( 0 C) Temp. at 75% wt.loss( 0 C) Residue% XNBR XNBR 0.05MWCNT XNBR 0.15MWCNT

32 Chapter 4 Fig TGA curves of XNBR and XNBR-MWCNT nanocomposites 4.3C.4 Differential scanning calorimetry (DSC) DSC analysis results of XNBR and its nano composites are given in Table 4.6. DSC analysis was performed in order to determine the effect of MWCNTon the glass transition temperature of XNBR. There was a small increase in T g on the addition of nanotube. This was due to the high interfacial area of interaction between the nanotube and the matrix which reduced the mobility of the polymer chain segments. This was a sign of compatibility between carbon nanotube and XNBR matrix. It is understood that the loading of MWCNT used were too small to produce a more significant change in T g. Table 4.6. T g values of XNBR and XNBR-MWCNTnanocomposites Samples XNBR XNBR.05 MWCNT XNBR 0.15MWCNT T g 126

33 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites 4.3C.5 X- ray diffraction analysis (XRD) XRD patterns of MWCNT, XNBR-MWCNT nano composite and XNBR are shown in Fig MWCNT displayed one diffraction peak at 2θ= corressponding to a basal spacing of 0.35 nm. This is the characterestic peak of MWCNT. Amorphous nature of XNBR was evident from its XRD pattern. The characteristic peak of MWCNT was absent in the nano composite.this showed exfoliation of nanotube and indicated efficient mixing of nanotube with the polymer matrix. Fig X-raydiffraction pattern of MWCNT, XNBR-MWCNT nano composite and XNBR 4.3C.6 Fourier transform infrared spectroscopy (FTIR) Fig.4.23 shows the FTIR of XNBR, XNBR MWCNT nano composite and MWCNT. Almost all the bands associated with the stretching of CH 2 groups were shifted to lower wave number in the nano composite. The peak at 969cm -1 attributed to stretching of butadiene double bond in XNBR was shifted to 964cm -1. The absorption band at 1440cm -1 due to OH bend in 127

34 Chapter 4 carboxylic acid in XNBR was shifted to 1436cm -1. These shifts might be due to the interaction between polymer chain and nanotube surface.the characteristic peak of CN at 2238cm -1 remained intact. The broad band due to OH stretch of hydroxyl group in MWCNT disappeared in the nano composite. In the IR spectrum of MWCNT the peak at 1558cm -1 corresponded to the stretching mode of C=C bond that formed the frame work of carbon nanotube sidewall [50]. This peak was present in the nano composite also but with a reduced intensity. This showed the interaction of the polymer chain with the C=C bonds in the nanotube. Fig FTIR spectrum of XNBR, XNBR MWCNT nanocomposite and MWCNT 4.3C.7 Scanning electron microscopy (SEM) Fig (A), shows the SEM images of the MWCNT dispersed on the surface of the nano composite. Latex blending had given a homogeneous and uniform dispersion of nanotubes abundantly on the surface. 128

35 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Fig.4.24 (B), (C) and (D) show the tear fractured surface of XNBR- MWCNTnanocomposites at different magnification. Absence of any agglomerates and void spaces in the rubber matrix, as seen at low magnification is a clear evidence of the good dispersion of nanotube in rubber. Because of the strong interaction between MWCNT and XNBR, the nanotubes were deeply embedded in XNBR. Thus MWCNT can carry stress throughout the rubber matrix leading to an effective reinforcement in the properties. Under high magnification short length of the nanotubes were visible. This might be due to the curved structure of the nanotube with one or both ends sunk in the matrix. (A) (B) (C) (D) Fig SEM images of (A) surface of the nano composite (B, C, D) tear fractured surface of the nano composite 129

36 Chapter 4 4.3C.8 Conductivity studies Fig.4.25 shows the variation of log of DC electrical conductivity of the nano composites against MWCNT loading. Conductivity of pure XNBR was of the order With increase of MWCNT loading conductivity value also increased and became 10-2 ie orders of magnitude higher than that of pure XNBR. At low filler concentration the conductivity remained very close to the conductivity of the pure, electrically insulating polymer marix, since the fillers occur only individually or in small clusters throughout the matrix [51]. Percolation threshold and drastic increase in electric conductivity was a result of continuous network of MWCNT in the polymer matrix. The conducting CNTs were either making physical contact between themselves or were separated by small gaps [52] across which hoping or tunneling [53] of electrons took place. Lorraine et al. [54] have dealt with the electrical conductivity of nano composites prepared from aqueous dispersion of conductive nanofillers (carbon black, antimony doped zinc oxide and carbon nanotubes) and latex [poly (vinyl acetate-co-acrylic) polydisperse latex/a poly(vinyl acetate) polydisperse latex/monodisperse poly(vinyl acetate) latex.] When the conducting carbon nanotubes in aqueous dispersion were mixed with the latex and allowed to dry a series of microstructure changes took place and the conductive particles became trapped in the polymer matrix. The micro structure of the matrix could force percolation to occur at a lower value by segregating the conductive particles to a restricted volume within the microstructure [54]. 130

37 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Fig Variation of DC conductivity with MWCNT loading. 4.3C.9 Conclusions MWCNT is good reinforcing filler in XNBR. Preparation of the nano composite by sonication of purified MWCNT in SDBS and latex blending technique gave a uniform dispersion of MWCNT in XNBR. This was observed in the SEM images of the nano composites. Absence of new peaks in the IR spectrum indicated there was only a strong interaction between the matrix and filler and no new bonds were formed in the nano composite. In XRD analysis the characteristic peak of MWCNT at was absent in the nano composite, showing exfoliation in the nano composite and confirmed the strong interaction between nanotube and matrix. Mechanical properties of the nano composite increased with a very small concentration of MWCNT. Tensile strength increased by 78%, elongation at break by 86%, modulus by 31% and tear strength by 19%. The incorporation of MWCNT in rubber matrix produced small enhancement in thermal properties. Electrical conductivity experiments revealed the presence of a percolation network at low filler loading, as low as 0.15 phr. 131

38 Chapter 4 4.3D Graphene Nanoplatelets in XNBR Latex Compared to carbon nanotubes, graphene nanoplatelets are cheaper and possess higher functionalities due to their high structural integrity[55] The two dimensional platelet geometry of graphene and graphene based materials offer certain property improvements which even single walled nanotubes cannot provide when dispersed in a polymer marix [56] Their high thermal conductivity, mechanical stiffness, fracture strength, electronic transport properties etc.can be harnessed when they are incorporated in a composite material [57]. Graphene based nanocomposites were prepared in XNBR latex. Graphene dispersion was made in a non ionic surfactant vulcastab VL. Graphene was added in small concentration ranging from 0-0.5phr. Mechanical properties of the nanocomposites increased even with the very low concentration of graphene.there were increase in the thermal stability and glass transition temperature of the nanocomposites. FTIR, XRD and FESEM analysis were used in the characterization of the nanocomposites. 4.3D.1 Mechanical properties Mechanical properties of XNBR-graphene nanocomposites are shown in Fig Fig. 4.26(A) gives the variation of tensile strength and elongation at break while Fig.4.26(B) gives the variation of modulus at 300% elongation and tear srength of the nanocomposite at various concentrations. It is seen that tensile strength, elongation at break, modulus and tear strength of XNBR increased significantly, reached an optimum value and then decreased.the tensile strength of the nanocomposite increased by 38%, elongation at break by 73% and tear strength by 27%, with a graphene concentration as low as 0.15 phr. 132

39 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites Strong interfacial adhesion between the platelets and polymer matrix was crucial for effective reinforcement [58-60]. Improved nanofiller dispersion, and alignment of the filler in the matrix also enhanced the mechanical properties of the nanocomposites. The tensile modulus of the nanocomposite showed an increase of 21% at 0.2 phr concentration. When dispersed in a polymer matrix, the thin platelets would adopt wavy or wrinkled structures.under the influence of an applied tensile stress these crumpled platelets would tend to unfold, rather than stretch in-plane and this reduced the modulus value [61]. Fig.4.26.Variation of (A) Tensile strength and Elongation at break (B) Modulus and Tear strength of XNBR-graphene nanocomposite 4.3D.2 Swelling studies Swelling studies were done in methyl ethyl ketone.the variation of solvent sorption Qt% against time 1/2 and swelling index (%) with concentration of graphene are shown in Fig.4.27 and 4.28 respectively. With the addition of filler the swelling rate decreased. This can be ascribed in terms of the polar nature of XNBR matrix. Polar-polar interaction between polymer and filler reduced agglomeration of filler 133

40 Chapter 4 particles. The swelling of 0.05 and 0.3 phr nanocomposites followed the same trend. Swelling was rapid at the beginning and then the swelling rate reached equilibrium. Fig Sorption curves of XNBR- graphene nanocomposites Fig Variation of swelling index with graphene loading 4.3D.3 Thermogravimetric analysis (TGA) TGA curves of XNBR and nanocomposite containing 0.15 phr graphene are shown in the Fig.4.29 and thermal degradation results are given in Table 4.7.The data revealed that there was an increase of 7 0 C in the onset temperature by the addition of 0.15 phr graphene. Temperature of maximum weight loss (T 2 ), temperature at 5% and 50% weight loss 134

41 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites were higher in the nanocomposite. Thus the addition of a very low concentration of graphene increased the thermal stability of the nanocomposite. The residual value of the nanocomposite was almost the same as that of XNBR. Fig TGA curves of XNBR and XNBR-0.15 graphene nanocomposite Table 4.7. Thermal analysis results of XNBR and XNBR-0.15 graphene Samples On set temp. Tmax ( 0 C) Temp. at Temp.at Residue ( 0 C) T 1 T 2 5%wt.loss( 0 C) 50%wt.loss % XNBR XBR Graphene D.4 Differential scanning calorimetry (DSC) Table 4.8 gives the T g of XNBR and the nanocomposites containing different graphene loadings. T g of XNBR without filler was at C. There was a gradual increase in T g with the addition of graphene. Thus with 1phr loading of graphene T g increased to C. This showed the incorporation of graphene even in very small concentration increased the stiffness of the composite and restricted the mobility of XNBR chain. 135

42 Chapter 4 Table 4.8. T g values of XNBR and XNBR graphene nanocomposites Sample T g ( 0 C) XNBR XNBR-0.05graphene XNBR-0.15graphene XNBR-0.2graphene XNBR-1graphene D.5 X- ray diffraction analysis (XRD) X-ray diffraction pattern of graphene and XNBR-0.15 graphene nanocomposite is shown in Fig.4.30.The broad peak of nanocomposite indicated exfoliation of graphene platelets by the intercalation of ploymer chain. Fig X-ray diffraction pattern of graphene and XNBR-0.15 graphene nanocomposite 4.3D.6 Fourier transform infrared spectroscopy (FTIR) Fig.4.31 shows the IR spectrum of graphene, XNBR-0.15graphene nanocomposite and XNBR. The peaks corresponding to the CH stretch vibration and CN group of XNBR were seen in the composite also. The 136

43 Carboxylated Nitrile Rubber Latex (XNBR) Based Nanocomposites peaks at 1539 cm -1 and 1437cm -1 giving the skeletal vibrations and the aliphatic CH bending [62] respectively of graphene sheets were seen to be more pronounced in the nanocomposite with a slight shift to a higher wave number. This showed the interaction of electrons of graphene platelets with the polymer chain. Fig FTIR spectrum of graphene, XNBR-0.15 graphene nanocomposite and XNBR 4.3D.7 Field emission scanning electron microscopy (FESEM) Fig shows the FESEM images of XNBR-graphene nanocomposite surface and Fig.4.33 represent the FESEM images of the tear fractured surfaces of the nanocomposite. A uniform distribution of graphene with clear particle profiles and clear interface boundary were seen on the surface of XNBR. While photographs of the tear fractured surface gave information on the interfacial adhesion between the filler particles and rubber matrix. In the tear fractured surface, the interface profile was indistinct showing good adhesion between filler and matrix [63]. 137

44 Chapter 4 Fig FESEM images of the surface of XNBR-graphene nanocomposite Fig FESEM images of the tear fractured surface of XNBR-graphene nanocomposite 4.3D.8 Conductivity studies. The DC electrical conductivity of XNBR-graphene nanocomposites are plotted in Fig.4.34 as a function of graphene loading. A sharp rise in conductivity was observed at 0.1phr concentration of graphene, indicating a filler network formation. The high aspect ratio of graphene enabled a good network formation in the matrix at such a low concentration of graphene. Generally percolation threshold and drastic increase in electrical conductivity results when the concentration of filler is sufficient to provide continuous electrical paths through the polymer matrix. The conducting elements should either make contact between themselves or should be separated by very small distances across which electrons could tunnel. The percolation threshold varies considerably with the shape and agglomeration of the filler 138